Phototube



Patented Dec. 29,` 1936 UNITED STATES PATENT OFFICE rno'ro'rUnE ofDelaware Application september 19, 1933, serial No, 690,045

4 Claims.

This invention relates to space discharge devices which respond tolight, and more particularly such devices in which photoelectriccurrents control the magnitude of a thermionic discharge.

Phototubes as usually made are in most of their practical applicationsused in conjunction with a vacuum tube amplifier. A high resistance mustbe used in the external circuit of the photoelectrodes to` obtain fromthe small photoelectric current sufficient voltage drop in theresistance to affect the bias of the grid of the amplifier to which thephototube is coupled, and the high impedance of the phototube causesdifiiculty in making an eflicient couplingbetween the phototube and theamplifier.

. To simplify the problem of amplifying a photoelectric current use hasbeen made of a combination tube comprising a phototube and a vacuum tubeamplifier mounted in one bulb, with the photocathode connected to thecontrol grid of the amplifier, and the photoanode connected to the anodeof the amplifier, but this combination tube has not been particularlysuccessful commercially. The structural arrangement is complicated, somekind of shielding arrangement between phototube and amplifier isnecessary, and a vacuum type phototube must be used because thethermionic amplifier is of the vacuum type.

One object of the present invention is to provide a method ofphotoelectrically controlling the output of a thermionic cathode in anoval way whereby a greater output is obtained at lower voltages than bythe methods heretofore available.

Another object is to provide a phototube in which the output of athermionic cathode is controlled by a. photocathode in a simple anddirect way and without the aid of a control grid or of high externalresistance.

A further object is to provide a phototube in which the current flowfrom a thermionic cathode in the same bulb as a photoelectrode iscontrolled by the effects produced in the atmosphere of the bulb by thephotoelectric current.

Still another object is to provide a high output phototube having a verysimple and strong electrode assembly.

In accordance with my invention amplification of the photoelectriccurrent is obtained by means of a thermionic cathode such as a straightfilament, a light sensitive photocathode adiacent the thermioniccathode, and a gridlike electrode interposed between the two cathodesand surrounding the filament, and acting as a common anode for bothcathodes. These three electrodes are mounted in a tube containing argonor similar gas at a pressure of to |25 microns such as is commonly usedin gas type phototubes.

In operation the three electrodes are maintained at different potentialsso chosen that the difference of potential between the grid anode andthe thermionic cathode is less than the ionization potential of the gas,but the potential diiference between the anode and the photocathode isgreater than the ionization potential. Under such conditions a smallchange in the emission of the photocathode causes a large change in thethermionic current to the anode. It is my belief that when the tube isin operation and as long as the photocathode is dark,

the current from the thermionic cathode to the anode is substantiallyfree from ionization and is space chargelimited. When light falls on thetube a photocurrent flows from the photocathode to the anode and ionizesthe gas to an extent dependent on the magnitudeof the photocurrent. Mostof the ions appear near the anode .where the speed of the photoelectronsis highest,

and hence are in the vicinity of the thermionic cathode. The positiveions partially neutralize the space charge adjacent the thermioniccathode, and as a result the thermionic current increases, and in effecta large amplification of the photocurrent is obtained.

'I'he invention willbe better understood by reference to the followingspecification when considered in connection with the accompanyingdrawing in which:

Figure 1 is a perspective View of one form of tube made in accordancewith my invention, parts being broken away better to show internalstructure;

Figure 2 is a perspective view of a modification of the tube, partsbeing broken away better to show internal structure;

Figures 3 and 4 are circuit diagrams of' practical applications of thetube; 1

Figure 5 is a curve showing the relation between photoelectric andthermionic currents in the tube.

The particular form of tube shown in Figure l has a cylindricalelectrode assembly comprising a thermionic cathode i, preferably a Vshaped lament coated with oxides, such as barium and strontium oxides.The cathode emission is not critical, but should be sufiicient to causethe current to be limited by space charge. The cooperating tubularelectrode 2, which is coaxial with and surrounds the thermionic cathode,operates as an anode, and is conveniently constructed like the usualgrid consisting of a helix wound on two side rods, so that electrons maypass through it. This grid electrode 2 may be either flat orcylindrical, and the diameter and turns per inch of the wire wound onthe side rods may be selected to give the tube the desired electricalcharacteristics. The grid-like or perforated anode 2 is surrounded by atubular photocathode 3, which is coaxial with the anode 2 and which mayalso be flat or cylindrical. The photocathode 3 may be made lightsensitive on its outer surface in well known ways, and I have obtainedgood results with the process generally used to make thin film caesiumoxide photocathodes. I prefer to make the photocathode of silver coatedsheet metal, and to permit operation of the tube independent of thedirection of the incident light. I may perforate the photocathode with aplurality of openings I. These openings 4 in the photocathode should belarge enough so that the electrostatic field in the opening will notprevent electrons emitted from the outer surface of the photocathodepassing through the opening to the anode 2. The filamentary thermioniccathode I is carried on leads 5 and 6, the anode 2 on leads 1 and 8, andthe photocathode 3 on leads 9 and I0, all these electrode leads beingcarried by the stem II which is sealed into the neck of a cylindricalbulb I2. The tubes should contain enough gas to produce appreciableionization at reasonable operating voltages, such as 90 to 100 volts.Good results have been obtained with tubes, made as shown in Figure l,which contained argon at the usual pressure of gas type phototubes suchas 100 to 125 microns.

The tube of Figure l is satisfactory for practical purposes, but forexceptionally accurate measurements the tube may be modified, as shownin Figure 2, to embody precautions to prevent the light from thefilament affecting the photocathode. The tube shown in Figure 2 has afilament I3, of straight fine wire, such as nickel, coated with bariumand strontium oxides. The upper end of the filament I3 is connected to awire Il, and both filament I3 and the wire Il are supported by leadwires I5 and I6. A cylindrical grid electrode or anode I1 having metalend collars I8 and I9 is mounted to surround and be coaxial with thecathode I3, and is supported by a lead wire 20 welded to the lower endcollar I9. A fiat photocathode 2I, of solid sheet metal with its surfacerendered light sensitive, is set edgewise to the filament I3, which liesin the plane of the fiat photocathode 2I. The only part of thephotocathode exposed to the light from the filament I3 is the edge 22.Additional shielding of even the edge 22 of the photocathode may beobtained by a thin narrow metal strip 23 mounted on the metal collars I8and I9 to lie in alignment with the filament I3 and to be out of contactwith the wire coil or helix of the grid electrode or anode I1. Thisnarrow strip shades or shields the photocathode 2i from the incandescentfilament I3. In a tube thus constructed the photocathode 2| is affectedonly by light which is intentionally projected upon its surface tooperate the tube. The photocathode 2l is carried by a lead wire 24, andthe lead wires for all electrodes are supported by the glass stem 25,which is sealed into the neck of the glass bulb 26 which encloses theentire electrode assembly. 'I'his tube operates in essentially the sameway as the tube shown in Figure l.

Tubes constructed as shown in Figures 1 and 2 have enough output tooperate a relay when connected as shown in Figures 3 and 4, in which thetube elements are represented diagrammatically. In Figure 3 thefilamentary thermionic cathode 21 is heated by a 1.5 volt filamentbattery rMythe perforated anode 29 is kept about 6 volts positive withreference to the cathode 21 by the 6 volt plate battery 30, and thephotocathode 3I is made negative with reference to both the filamentarycathode 21 and the anode 29 by the 45 volt battery 32, which has itspositive terminal connected to the filament 21 and also to the negativepole of the anode battery 30. the tube connected as shown in Figure 3,there is no photoelectric current in the tube, but a thermionicdischarge, too small to operate the relay 33, fiows between thefilamentary cathode 21 and the anode 29. As the voltage drop of about 6volts between the cathode 21 and the anode 23 is less than theionization potential of the gas in the tube, the discharge between themis free from ionization and is limited by space charge.

When light falls upon the light sensitive cathode 3I photoelectrons areemitted by it and travel toward the anode 29 and the thermionic cathode21, both of which are positive with reference to the photocathode. Thevoltage drop between the photocathode and the anode 23, due to the twobatteries 30 and 32, isgreater than the ionizing potential of the gas inthe tube, hence the photoelectrons fiowing to the anode 29 and filament21 ionize the gas and thus produce positive ions. The number of positiveions thus produced is dependent on the number of photoelectrons emittedby the photocathode, and these positive ions neutralize to an extentdependent on their number the electron space charge between the filament21 and the anode 29. A comparatively small photoelectron current fromthe photocathode 3I will produce enough ions to have a marked effect onthe space charge and thereby cause a comparatively large thermioniccurrent to fiow from the filament 21 to the anode 29 whereupon the relay33 operates and the work circuit 34 is closed.

Figure 4 illustrates an adaptation to a picture transmission,television, or other like circuit of a phototube embodying my inventionand operating as explained in connection with Figure 3. The circuit ofFigure 4 differs from that of Figure 3, in that, the l5 volt platebattery 35 Is of a higher potential than the plate battery 33 in Figure3, and that the relay 33 in the anode circuit is replaced by a couplingtransformer with its primary 36 in the anode circuit and its secondary31 connected to the first tube of a thermionic amplifier. The outputimpedance of my photoamplifier tube is comparatively low, hence theimpedance of the transformer is correspondingly low, and need notnecessarily be higher than 2000 ohms. Such a low impedance transformeris much less expensive and much less susceptible to electricaldisturbances such as "pick-up" from power lin or the like than the highimpedance transformer necessarily used with the usual phototube.

Figure 5 shows a curve 33 obtained from one tube made in accordance withthis invention and showing the change produced in the thermionic currentflowing between the thermionic cathode With no light on the photocathode3l of.

andanodeinresponsetoachangeinthephoto-u of the light falling onthephotocathode.

electric current due to variation in the intensity In this tube aphotoelectric current of one microampere corresponds to a thermioniccurrent of approximately three milliamperes, as shown by the dotted line39, and an increase in photoelectric current to three microamperescauses the therimionic current to increase to 4.2 milliamperes, as shownby the dotted line 40. The amplification of the photoelectric current isso great that in many cases my tube will replace the usual combinationof a phototube and glow tube, or phototube and external amplifier, andwill produce much the same results by a simpler and less expensivedevice.

It is to be understood that the embodiments of the inventionhereinbefore described are merely illustrative and that many changes maybe made without departing from the scope of the invention which islimited only by the appended claims.

What is claimed is:

l. A photoelectric current ampliij'ier comprising a sealed glassenvelope containing gas at reduced pressure and enclosing a thermioniccathode, an anode, and a photocathode external to said cathode andanode, an output circuit between said thermlonic cathode and said anodewhich includes means for impressing upon said anode a potential which ispositive relative to said thermionic cathode and leas than the ionizingpotential of the gas, and means for maintaining between said anode andsaid photocathode a constant difference of potential 'greater than theionizing potential of the gas.

2. Means for amplifying electric currents comprising a sealed glassenvelope containing gas at reduced pressure and enclosing a thermioniccathode, a source of electrons, and an anode common to both said cathodeand said source, means for maintaining between said anode and saidsource a constant difference of potential greater than the ionizingpotential of the gas, means for maintaining between said anode and saidthermionic cathode a constant difference of potential less than theionizing potential of the gas, and an output circuit between saidthermionic cathode and, said anode.

3. A photoelectric/ current ampliiler comprising a sealed glass envelopecontaining gas at reduced pressure and enclosing a thermionic cathode, aphotocathode, and an anode mounted to cooperate with. both saidcathodes, an output circuit between said anode and said thermioniccathode, means for maintaining both said anode and said therriioniccathode positive with reference to said photocathode at potentials suchthat the potential of said anode to said photocathode is greater and tosaid thermionic cathode is less than the ionizing potential of the gas.

4. Means for amplifying photoelectric currents comprising a sealed glassenvelope con'- taining gas at reduced pressure and enclosingA athermionic cathode, a photocathode, and an anode common to both saidcathodes, an input circuit including said photocathode and said anode,an output circuit including said thermionic cathode and said anode, anda source of potential greater than the ionizing potential of the gaswith its negative terminal connected to said photocathode, anintermediate point at a positive potential lower than the ionizingpotential of the gas connected to said thermionic cathode, and itspositive terminal connected to said anode.

HERBERT NEISON.

